Variation of the human menstrual cycle through reproductive life.

30 years of data directed to the study of the temporal characteristics of the human menstrual cycle and some of its associated phenomena are presented. Approximately 2700 record-keeping collaborators and their daughters were enrolled. Only records of greater than 6 months uninterrupted recording are included. A record of all surgery illness or medication which might influence menstruation was recorded as well. Age of the patient and age since menarche are recorded. By 1961 this study had accumulated 25825 person-years of menstrual experience with 2750 intervening pregnancies. Justification of the belief that women normally vary in menstrual interval around a value of 28 days was unsupported. Each woman has her own central trend and variation which changes with age. The menstrual interval should be expected to average within a few days of 28 when assembled in large groups. Variation as opposed to regularlity is the rule in the menstrual cycle. The early and late years of menstrual life have the greatest variation in pattern of both long and short cycles and tend to mirror each other. The middle years of menstruation is characterized by linear change in both central tendency and variation; falling 2-3 days in this age span. Variation in menstrual cycle reaches its minimum at around 36 years of age. Determining the pattern of variation in menstrual history for anticipated intervals needs further study. The practice of birth control through the use of rhythm methods alone must introduce elements of uncertainty which are serious problems.